Truss modeling in RISA

[PBEIT]

I am modeling some existing existing truss in RISA. I have always thought that truss members should be modeled with pin connections. The existing truss is completely welded. So, should I still model this as pin connected? Also, I only have hand sketches of these trusses and in one of them all of the diagonal web members are in the same direction with vertical members between them like a howe truss with all the diagonal members in the same orientation. I know that truss webs usually are triangle for tension and compression but even with welded connections it just looks like it the parallel top and bottom cords would not be adequately supported by the web members. Thoughts?

 

[JedClampett]

It's conservative to model the truss with pinned connections. If you model it as fixed, you'll have to check all those welded connections and members for moments.
As far as the second question, the truss will be fine, but your verticals will see forces (stresses). For other trusses many of the verticals are zero force members.
No harm in doing this truss by hand. It's actually pretty easy. Then use RISA to check your work.

 

[msquared48]

RISA gives you the ability to model the truss as it is an get real world results. Why wouldn't you take advantage of that?

Mike McCann, PE, SE (WA)

 

[PBEIT]

Never mind the part about all the diagonal web members being in the same direction. It is a 100' Howe truss made from two 50' sections, making it a regular good ol' Howe truss. Thanks for you quick responses.

 

[Lomarandil]

I'm with Mike. If the connections are reasonably beefy, I tend to model the truss as fixed-end, then spot check a few members and connections that develop bending to make sure they're alright. Typical trusses will still develop mostly axial forces, except where geometric irregularities are present (for example, where the lateral force system or floor system frames in).

I can see how it would be conservative for the members to consider everything pinned. I'm not sure whether that would be conservative for the connections or not.

 

[msquared48]

Actually, it may not be conservative if the top and/or bottom chords are heavily loaded. T/C +M considerations here...

Mike McCann, PE, SE (WA)